1379957 - DOM workers continue to run idle GCs every few seconds when idle

Status: RESOLVED FIXED

(Core :: DOM: Workers, enhancement)

Description

[Jon Coppeard (:jonco)]

Something I noticed a while ago is that DOM worker threads periodically GC even when the browser is totally idle due to IdleGCTimerCallback running repeatedly.  

I tracked this down to the DebuggerOnGCRunnable which is enqued from CycleCollectedJSRuntime::GCSliceCallback.  This happens at the end of GC and makes the worker run loop think that the worker is active again and restarts the idle GC timer.

Ideally we would ignore events associated with GC when deciding whether the worker is idle but I don't know how feasible that is.  A simpler mitigation would be to skip enqueuing this runnable if there were no debuggers active.

Comment 1

[Jon Coppeard (:jonco)]

Nick, any ideas on how best to proceed?

Comment 2

[Ben Kelly [:bkelly, not reviewing]]

> A simpler mitigation would be to skip enqueuing this runnable if there were no debuggers active.

This one sounds more appealing to me, FWIW.

Comment 3

[Ben Kelly [:bkelly, not reviewing]]

I think Olli was trying to make some worker GC changes recently as well.  He might have opinions.

Comment 4

[Olli Pettay [:smaug][[e-mail]]]

GC/CC scheduling on workers is totally busted, and when I tried to fix it, it always caused test failures :(

But definitely debugger code shouldn't make the behavior worse.

Comment 5

[Nick Fitzgerald [:fitzgen] [⏰PST; UTC-8]]

(In reply to Jon Coppeard (:jonco) from comment #0)
> A simpler mitigation would be to skip enqueuing this runnable
> if there were no debuggers active.

Yeah, there's no point in posting this runnable if there aren't any debuggers observing any global within the set of zones being collected.compartment

Comment 6

[Jon Coppeard (:jonco)]

Attachment: bug1379957-only-fire-ongc-hook-when-necessary

Patch to check whether there are any debuggers that will have their onGarbageCollect hooks fired before dispatching the runnable.

Comment 7

[Nick Fitzgerald [:fitzgen] [⏰PST; UTC-8]]

Comment on attachment 8885717 [details] [diff] [review]
bug1379957-only-fire-ongc-hook-when-necessary

Review of attachment 8885717 [details] [diff] [review]:
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Looks great :) Thanks!

Comment 8

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/872e9c8fec61
Only fire the debugger's onGarbageCollection hook when necessary to avoid extra worker GCs r=fitzgen

Comment 9

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/872e9c8fec61